1. Field of the Invention
The present invention relates to an improvement of a control circuit that instructs a pulse generating circuit used in a discharge-lamp lighting device to generate pulses.
2. Description of the Related Art
Discharge lamps require lighting devices also called electronic ballasts. The lighting devices generate high-voltage pulses to start an electric discharge, that is, dielectric breakdown between electrodes of the discharge lamps when starting the discharge lamps, and also prevent excessive electric current from passing through caused by a drastic decrease in the voltage between the electrodes (lamp voltage) after the discharge lamps are turned on.
In addition to the above described roles of the lighting devices, to prevent the discharge lamps from flickering, high-frequency lighting and rectangular-wave lighting have been widely used in recent years. To implement these types of lighting, electronic discharge-lamp lighting devices in which a step-down chopper circuit and a bridge circuit are formed by using semiconductor elements to generate the desired high-frequency or rectangular-wave signals have been often used.
To start a discharge lamp, a lamp-voltage detection section first detects the lamp voltage; and, when it is determined that the lamp voltage is at a no-load level, a microcomputer instructs a pulse generating circuit to generate high-voltage pulses. Japanese Unexamined Patent Application Publication No. 2003-100488 describes a discharge-lamp lighting device in which the voltage and current of an electric power converter are detected, a trigger signal is input to a transistor of a high-voltage generating circuit formed of a transformer and the transistor, and an starting high voltage is applied to the lamp.
FIG. 3 shows an example of a conventional discharge-lamp lighting device. This lighting device includes a power-factor correction circuit 14, a step-down chopper circuit 16, a full-bridge circuit 18, and a pulse generating circuit 20. The lighting device also includes a control circuit 124 that operates with a DC voltage supplied from a control power supply circuit 26. A lamp-voltage detection section 132 provided for the control circuit 124 detects an analog value of the lamp voltage and gives the value to a pulse-generation command section 134. A lamp voltage sufficiently lower than a no-load voltage is detected during stable lamp operation. If a lamp 22 goes out during stable operation, the detected lamp voltage increases up to the no-load voltage. The pulse-generation command section 134 determines, according to the detected value of the lamp voltage, whether the lamp is in a no-load state (going out of lamp), and if it is in a no-load state, the pulse-generation command section 134 issues a command to the pulse generating circuit 20 to generate pulses. Restarting the lamp is performed in this manner when the lamp voltage at a no-load state is detected.
After the lamp is turned on correctly, the lamp voltage levels off at a level sufficiently lower than the no-load voltage when the lamp has been used for a short time. If the lamp has been used close to the end of its life, however, the lamp voltage goes high during stable lamp operation and approaches the no-load voltage. Therefore, the analog lamp voltage detector may erroneously detect the no-load voltage even during stable lamp operation if the lamp voltage merely becomes close to the no-load voltage, causing the generation of unwanted starting pulses. This may lead to lamp flickering or lamp malfunctioning.